1. Field of the Invention
This invention relates to a printer, and more particularly, to an apparatus for feeding a printing media, such as a printing sheet or paper, for printers having a plurality of printing heads arranged along the feeding direction of the printing sheet and a printing sheet driving means comprising a speed reduction gear train including reduction gears. This invention also can be applied to a drive means for driving a photosensitive drum or belt of a printer.
2. Description of the Related Art
A conventionally known printer includes a cylindrical platen for feeding a printing sheet and a plurality of printing heads arranged along the periphery of the platen for printing various colors in turn, for example, yellow (Y), magenta (M), cyan (C), and black (B), respectively, to obtain a full color image on the printing sheet. A large gear is directly mounted on the platen and driven by a step motor via a speed reduction gear train including gears.
In the conventional printer, however, the relationship between the distance from one of the printing heads to the adjacent head, and the number of revolutions of the reduction gear or gears has not been particularly considered.
These reduction gears, however, are always involved in pitch errors due to slight aberrations when these gears were made by a mechanical process. Therefore, if a speed reduction gear train including gears is employed, the pitch errors of several gears are accumulated, so that the platen is rotated with unstably with pitch errors.
For example, if four printing heads for different colors of yellow (Y), magenta (M), cyan (C), and black (B) are arranged along periphery of the platen, the pitch errors are generated in the respective printing heads as shown in FIG. 4. For example, at a printing position A, the printing of magenta (M) is incorrect in the forward direction, and on the other hand, the printing of cyan (C) is incorrect in the rearward direction. Namely, there are two kinds of pitch errors, i.e., pitch errors in the forward and rearward directions, and therefore, a maximum amount of printing aberration between two printing heads is represented as double the accumulated pitch errors.
The aberration of the printing position due to the aberration of the gear pitch generally causes an uneven darkness in the feeding direction, but a small aberration of the printing position due to pitch errors of the gears particularly causes a color aberration.